Hydraulic motor control system



May 4, 1965 R. A. MAGNUsoN HYDRAULIC MOTOR CONTRGL SYSTEM 2Sheets-Sheet. 1

Filed Jan. 25, 1963 Illini "lll INVENToR. R slanci A-m May 4, 1965 'R.A. MAGNUsoN 3,181,427

HYDRAULIC MOTOR CONTROL SYSTEM Filed Jan. 25, 1963 2 Sheets-Sheet 2 59IN1/EMDR.

.EL-.il and: A.Tl'laqhusun.

United States 3,18L27 HYDRAULEC MGTR @@NTRL Roland A. Magnuson, Seattle,Wash., assigner, by ruesne assignments, to the United States of Americaas rep-resented by the Secretary of the Army Fiied lan. 2S, i963, Ser.No. 254,626 8 Claims. (Cl. gif-292) This invention relates in general tocontrol systems for variable displacement hydraulic motors used indriving a load and in particular to gun control systems in which thedisplacement of the hydraulic motor is automatically adjusted to suitvarying torque requirements encountered in depressing, elevating ortraversing the gun.

Modern armament requires large caliber guns to be mounted on vehicles,and large expenditures of energy are required to manipulate such guns.This energy is usually :supplied by batteries or other components whichhave limited amounts of energy available and have to be transported bythe vehicle. These batteries operate hydraulic components to elevate andtraverse the Weapon. The amount of energy necessary to move the gunvaries with vehicle inclination. On level ground the torque at thetrunnions, required to raise and lower an equilibrated gun, is fairlyconstant throughout the entire range of elevation. However, placing thegun platform on an inclination such as a rising slope will greatlyincrease the torque necessary to depress the gun and greatly decreasethe torque required to elevate the gun. A similar torque increase isalso required to traverse the gun on such a slope. Obviously, thegreater the slope, the greater are the torque requirements.

Prior to the present invention, vehicle mounted guns have usually beenmanipulated With a constant pressureconstant displacement hydraulicmotor system in which the motor has to be geared to the gun with such aratio that the torque at the trunnions developed by the motor Will equalthe maximum torque load which has to be overcome. Each turn of the motorconsumes a tlXed volume of iluid, and each volume of iluid contains axed amount of pressure energy maintained by a hydraulic pump unitoperated by an electric motor. This motor is energized by batteriescarried by the vehicle. Such motors run at maximum etliciency only whenthe torque developed by full system pressure is just suicient toovercome the torque load. At lesser loads such as those encountered whenthe vehicle is on level ground, at the same motor speed, the motorconsumes the same quantity of iiuid and requires the expenditure of thesame amount of energy as when heavier loads are moved. However, only anamount of energy equal to the lesser load is delivered. The energy usedis dissipated in throttling in a control valve or the motor wouldaccelerate to a higher speed. When moving low loads at high motor speed,energy is Wasted at a high rate since larger volumes of fluid than arenecessary are used to move the load.

A system which utilizes a variable displacement motor with thedisplacement manually controlled so that the motor torque Wouldequal theload torque and would economically use the available duid. However sucha system would be impractical because constant manual manipulation ofthe control is required and there would be undesirable acceleration anddeceleration properties near the zero stroke point of the motor.Furthermore, with such an arrangement considerable operator trainingwould be necessary for successful and economical operation.

it is therefore an object of the present invention to provide ahydraulic system having a constant pressure source and a variabledisplacement motor for moving varying loads in which the displacement ofthe motor is auto- Edeka?? matically adjusted to suit the varying torquerequirements of the load.

A further object of the invention is to provide a gun control systemwhich etliciently uses the pressure energy supplied to a variabledisplacement hydraulic motor in traversing and elevating the gun.

Another object of the present invention is to provide a gun controlsystem in which the hydraulic motor used to move the gun is arranged tocontrol its own stroke.

Another object of the invention is to provide a gun control whichaccurately aims the gun at a Xe-d or slowly moving target and whichrapidly manipulates the gun when necessary.

Objects and advantages other than those set forth will be apparent fromthe following description When read in connection with the accompanyingdrawing in which:

FGURE l is a schematic diagram partly in section and not to scaleindicating essential elements of the hydraulic gun control system.

FlG. 2 is a schematic diagram similar to FlG. l Which illustrates thevalve spool in the system being displaced from the position in FIG. l topermit iluid flow within the system.

FlG. 3 is a side elevational view of a fragment of the valve spool ofFlGS. l and 2 showing one of the lands.

FIG. 4 is a top view of FlG. 3 taken along line 4 4 of FlG. 3.

FG. 5 is a diagrammatical cross-sectional view of the variabledisplacement hydraulic motor and the associated stroke control cylinder.

FIG. 6 is a side elevational view of the hydraulic motor utilized in thesystem.

FIG. 7 is a sectional view of FIG. 6 taken along lines 7 7 of FIG, 6.

PlG. 8 is a cross-sectional view of the stroke control cylinder of FIG.6.

As shown in FIG. l, a gun l having trunnions 3 which may be mounted forrotation in a traversible turret mounted in a vehicle or other support.Secured to the underside ot the gun is elevating segment gear S. The gunis actuated by gears and lll with gear 9 being secured toy the outputdrive shaft 6&9 of a variable stroke hydraulic motor l which is fixed inthe vehicle. Motor r3 is connected by conduits or tubular tluidilovvlines 33, 35, 37, 33 to any suitable hydraulic pump unit l5 whichprovides a constant fluid pressure source for the system. A reservoir 4is connected in line 33 between valve 17 and pump unit l5. The pump unitincludes a suitable conventional constaat displacement pump which isdriven by an electric motor lo connected to the vehicle electricalsystem by Wire conductors l. Connected to the fluid conducting linesbetween the pressure source and thc motor is a valve housing 17'. heinterior of valve housing lt is formed with a cavity l in which valvespool 2l is mounted for reciprocal longitudinal movement. Suitableoriiices including orilice 2u are provided in the valve housing toConnect cavity E9 to the fluid conducting lines. The valve housing hasalso formed therein iluid ilovv passages 23 and 2S connected to thecavity for a purpose to be later described. Resilient rings ld incompartments in each end of housing l' surround the spool and preventiluid leaks between spool 2l. and housing il?. The spool 2l is formedwith lands 2'?, 229 and 3d which lit closely in the housing and dividecavity i9 into four separatevchambers. Lands 2'?, 2S* and 3l preventdirect fluid flow between these chambers and when positioned in front ofthe orifices in housing l? for conduits 33, 35 and 37 prevent fluid ilowin these conduits and in the housing. The spool 2l may be springcentered in cavity l@ by coil `springs 7 so that the lands completelyclose these last mentioned housing ori- The lands may be tapered,cylindrical or any shape to t the internal dimensions ot cavity i9.

Y j, Y As best shown in FIGS. 3 and 4, the lands 27, Z9 and 31 arecylindrical in shape and are formed with dowi'iwardlyv inclined iiats 23on the outer surface of each land. These flats control the iiuid flowwithin the system and are proportionedV so that the amount of fluidpermitted to flow in housing 17 is proportional to the amount of valvespool displacement. Instead of dats 28 suitable metering notches (notshown) may be cut into lands 27,29, 31 to control the orifice openingsin the housing 17V for conduits 33, 35 and 37. Thus, as each valvespoolland is Y displaced away from its centered position of FIG. l, the

Vto bypass'valve housing 17 and return fluid displaced by motor to asuitable reservoir in unit 15 when lands 27, 29, 31 suficiently limitthe amount of fluid flow in valve housing 17.

FIG. 2 shows the valve spool displaced to the right in the direction ofthe arrow by handle 22. a When this displacement .is made, land 29 hasbeen moved to completely open the orifice in housing 17 for pressureconduit 37 and lands 27 and 31 completely open the oriiice for conduits33 and 35. Passage 23 permits the uid to flow in valve housing 17bypassing land 29, and into conduit 33 to operate hydraulic motor 13 forcounter clockwise rotation of :shaft 69 as viewed in FIG. 5. Since, asmentioned above, land 27 has also been moved, conduit 35 returns thefluid to the valve cavity 19. Passage in the valve housing and conduit38 provides the means for returning the iiuid back to the reservoir inthe pump unit 15. This path of uid dow is shown by arrows in FIG. 2.

If the spool is displaced an appropriate distance tothe left, the How ofduid will be reversed.. Lands 27, 29 and 31 in FIG. 2 will be positionedto the left of orifices in housing 17 serving conduits 34, 43 and 33,respectively. When the valve spool is so displaced fluid will enter thevalve housing from conduit 37 around land 29 via passage Z3 into conduit35 to motor 13. Conduit 33 then permits the fluid to return from themotor to the valve housing. The returning duid ows through passage 25and then through conduit 3S and reservoir 4 back to pump unit 15. Themotor drive shaft 69 of FIG. 5 will rotate clockwise. Thus, gun 1 willbe elevated or depressed depending on the direction of fluid flow withinthe conduits 33, 35. A hollow cylinder 39 iixed in the vehicle or to themotor 13 which controls the stroke of the motor is connected to thevalve housing by uid conducting conduit 43 and to the pressure conduit37 by conduit 41. As shown in FIGS. 1, 2, 5 and 8, a piston isreciprocally mounted in this cylinder.y The piston includes cylindricalhead 47 which fits closely in the cylinder 39 and is formed with anoriice 49 (FIG. 8) which permits the passage of the duid from one sideof the piston head to the other when the fluid flow is low. Suitablespacers such as the ring like spacer of FIG. 8 or the projecting :spacer40 of FIG 5 may be iixed in the cylinder to limit the movement of piston47 in cylinder 39 as it approaches cylinder head 39' which forms one endof the cylinder. This limits the movement of bearing 65 and establishesthe zero stroke of the motor. As shown in FIG. 8, head 39' ofcylinder 39has orifice 43' for conduit 43 and cylinder 39 has orifice 41' forconduit 41. Connected to the piston head is a rod or armA 51 whichextends outwardly from one endof the cylinder. An arm 53 pivotallyconnected to the piston arm by pin 52` extends to the extension 57 onmotor bearing 65 in hydraulic motor 13. A pin 55 pivotally connects the`arm 53 to extension 57 integrally formed on pivoted motor bearing 65.FIGS. 7 and 8 show the con- Shania? loA nection between the variabledisplacement motor and the piston. `The motor bearingris pivotallymounted by pin 61 to the motor housing 59 iixed in the vehicle as shownin FIG. 5. The bearing 65 is used to move the roller bearing race 63eccentric to the cylinder 67. FIG. 5 shows the bearing moved to itsextreme position for maX- imum motor stroke.

The variable displacement hydraulic motor 13 is adapted to be actuatedby the fluid iiowing from the pump through the valve housing 17.Thermotor 13 is provided with a rotatable cylinder 67 mounted in themotor housing on ball bearings 64. The cylinder has an integrally formeddrive shaft 69 connected through suitable gearing to theV gear 11meshing with the gun elevating arc 5. The cylinder has a plurality ofgenerally radially extending bores 71 in which are mounted forreciprocation small pistons 73. Axial ports 72 in the cylinder 67communicate with bores 71.V At their inner ends the ports 72 are adaptedto communicate with ports 75 and 77 provided in stationary fiat valve79. The ports 77 and 75 are crescent shaped and are adapted tocommunicate with the axial ports 72 in the cylinder as the cylinderrotates and the ports pass through the arc of ports 77 and 75. Backuppiston springs 83 hold tubular backup pistons 85 in head 87 against themotor flat valve 79. Two equalizer pistons (not shown) in the iiat valvebalance the hydraulic forces which tend to separate the dat valve 79from the ported end of motor cylinder 67.

The bearing race 63 is rotatably mounted in the pivoted bearing 65 uponball bearings 31. When the bearing 65 is moved a sufhcient amount by arm53 the bearing race andthe cylinder 67 will be eccentric with respect toeach other to provide piston stroke.

n Counterclockwise rotation of the cylinder and drive shaft 69 isaccomplished when the valve spool is displaced as in FIG. 2 and oilunder pressure is furnished by the constant displacement pump to themotor through the valve housing 17. The oil liows out of the valvehousing via conduit 33 to port 33 (FIG. 7) in motor head 87.

`Oil entering port 33' flows through the head, hollow upper backuppistons 85, through suitable passages in valve 79. The oil then ows inupper crescent 75 through axial ports 72 in the motor cylinder 67 toradial bores 71 to force the pistons 73 passing through the arc of theupper motor flat valve crescent outward. This causes the inner race 63of bearing 65 and the cylinder 67 to rotate counterclockwise as viewedin FIG. 5. Oil is discharged by the motor rolling pistons 73 as thepistons pass through the arc of the lower valve crescent. The oil isforced by the pistons into axial ports 72 in the cylinder 67 andV intolower crescent 77 through passages in the dat valve 79. The dischargedoil then flows through the lower backup pistons and out of port 35' inhead 87 to valve housing 17 via conduit 35. The pistons 34 aremaintained in contact with the roller bearing race 65 by centrifugalforce plus hydraulic pressure. By reversing the duid flow the cylinder67 and its vdrive shaft will have clockwise rotation. Increased motorstroke is obtained by increasing the eccentricity of race 63 withrespect to cylinder 67. rEhe operation of such piston drivenhydrodynamic motors can be found in U.S. Patent No. 2,406,138 to W.Farris et al.

Low flow operation;

When the operator desires to aim the weapon at a stationary or slowlymoving target the weapon must be moved at low speeds for accuratepositioning. Depending on direction of target movement, the valve spoolis manually displaced Vfrom the center position in either direction toopen the orifices in the housing controlled by the lands from 0 to 75%of the total possible orifice opening. Since the amount of fluid ow invalve housing 17 is controlled by lands 27, 29, and 31, land theirassociated ats 28, there will be a relatively low volume of fluid perunit time or low ow of fluid through the system and the motor speed islow. At low flows orifice 49 short circuits land 29 to permit fullsystem pressure on both sides of the stroke control piston. This shortcircuit occurs because the opening 2li in the valve housing is onlypartially opened by land 29 and this partial opening is small whencompared to the opening 49 in the stroke control piston. Since the areaof the head end of the piston is greater than the area of the rod end ofthe piston (due to rod Si), the opposing forces on the piston head areunequal and the piston is moved to the left in FIG. 5, and the hydraulicmotor bearing is moved to a stationary position where there is maximumstroke in which the maximum amount of huid is displaced for each turn ofthe motor. This is desirable for tine control of the Weapon under allconditions. Motor speed and the velocity at which gun Si is moved iscontrolled by controlling the amount of oil which flows in the valvehousing passages tor the operation of motor i3. The llats or meteringnotches described above which are cut in the valve spool lands areproportioned so that luid flow and weapon velocity are proportional tothe displacement of valve spool 2l. Thus, when the valve spool has beendisplaced so the oriiices are open, the weapon moves with a low speed.With lesser openings, the gun would move with a proportionally lesserspeed. During the low ilows, the stroke of the motor is tired. It thestroke of the motor were not liked, but assumed a value dependent on thesize of the load, then a given valve displacement might result in a highgun velocity in one instance and in a low gun velocity in anotherinstance. This characteristic would not encourage accurate Weaponpositioning. Thus, for low tlows tired maximum motor stroke is desirableto obtain a system which allows accurate positionin(Y of the gun. Sinceline control only accounts for a minor portion of the total gun motionand since few turns of the motor are occupied by tine control, there isno extravagant expenditure of oil from the pressure source.

High How operalon When the operator desires to move the weapon fromloading to firing position or from one target to the general area ofanother target, it is obviously desirable to move the weapon at a highspeed. Since this mode of operation accounts for the bulk of gun motionand for the bulk of oil expenditure from the pressure source, highefficiency is desirable. Rapid gun motion is accomplished by displacingthe valve spool so that the valve orifices are from 75% to 100% open.Such valve spool displacement permits moderate and high iloW in thesystem as shown in FEG. 2. Such high fluid llovv results in high motorspeed. At moderate or high flows, there is no short circuit of land Athrough orifice .9 due to the fact that at this spool position theopening 2lb in the valve housing is large when compared to the openingi9 in the stroke control piston. The motor stroke automatically adjustsso that a constant and high percentage of the system pressure is alwaysapplied to the motor and also adjusts so that the torque generated bythe motor always equals the load torque. This results in the mostellicient use of the energy used to opcrate pump unit l5. In theequilibrium condition (FlG. 2), 5% of the system pressure is across land27, 5% is across land 29, 5% across land 3l and 85% is across the motor.100% system pressure is applied to the rod end of the stroke controlpiston and of system pressure is applied to the head end. Because thestrolte control piston has an area difference of 5% between the rod endand head end (due to the rod area) the forces on each side of the pistonare equal and the stroke stays where it is. With the above describedsystem motor stroke is automatically adjusted to meet dierent torquerequirements. For example, if the torque load increases such as wouldoccur ir" the vehicle mounting the gun was moved from a horizontalsurface to an inclined surface and the gun Was manipulated, the motorwill slow down and a higher percentage of system pressure is necessaryto run the motor, for example 95%. Then each of lands has 12/3% systempressure across it. The stroke control piston has system pressure acrossthe rod end and 81/3 across the head end. Since the piston is notbalanced, the piston moves bearing race e3 to the right in liiGS. l and2. Motor stroke is increased and so is motor torque, until 85% of systempressure again is exerted on the motor. The piston will be stabilized inits new position, the motor stroke and torque will be increased and themotor moves the load by using a high percentage (85%) of the systempressure. Thus, only 15% of the oil energy is dissipated in the controlvalve. lt is thus clear that the motor economically uses the energysupplied to it for driving various sized loads which occur in theelevation and the traversing of a Weapon. ln other Words, by controllingthe torque of the motor, it is not necessary to increase the output ofthe pump to supply increasing quantities of oil to the motor to make upfor increased loads. Therefore energy for running the lsump is conservedand is economically used.

This tact is illustrated in the table beloul in which there is acomparison of the present invention and prior art power requirement toelevate a mm. gun from a 15 loading angle to a 65 elevation and back to15 at a rate of 3 cycles per minute.

It is clear that batteries used with the present invention will lastmuch longer than those of the `prior art constant pressure-consantdisplacement motor system. This relieves the demand on not only thebatteries themselves but on battery charging devices and the amounts offuel necessary to operate those devices.

ln the automatic stroke control mode, gun velocity is only under verygeneral control because the motor stroke is automatically adjusted sothat motor torque equals the load torque until 85% of system pressureappears across the motor. Thus, if the gun is Well balanced or tends torun in the direction it is being moved, the motor stroke will be minimumand gun speed will be high. lf the gun resists the desired motion, motorstroke will adjust itself to a larger value, and gun speed will belower. In both cases, the quantity of oil which hows in the system isthe same and for any given oil flow the speed of the motor may vary by afactor of 5 or more depending ori' torque load.

The piston rod motion controls the stroke of the hydraulic motor.Maximum stroke occurs when the motor displaces the maximum amount offluid for each turn of the motor. The torque developed by the motor issimply:

Where e is the motor torque eciency, P is the applied pressure, l) isthe displacement per turn, and T is the torque developed. From theformula above, it is seen that by controlling the stroke, and therebythe displacement per turn, the torque is controlled.

it is contemplated that a single reasonably constant pressure source maybe utilized for the simultaneous operation ot several variabledisplacement hydraulic motors. Thus, for example, the pump unit ll5provides not only the constant pressure source to operate motor i3 toelevate and depress gun ll but also to operate a second variabledisplacement hydraulic motor for traversing the gun.

- to said source, iirst while a second gunner has two valves, one totraverse the gun and a second to control elevation if necessary.

snai/tavr AAlthough i have shown and described certain embodi- Y mentsof the invention, it will be understood that Ido not Wish to be limitedto the exact construction shown and described, but that'various changesand modifications may be made without departing from the spirit andscope of my invention, as provided in the appended claims. What I claimis: Y 1. In a constant pressure-variable displacement hydraulic motorsystem for moving a load, a Valve in said systemV forv controlling thequantity or" fluid per unit time which moves in said system, a strokecontrol cylinder, a piston mounted for reciprocation in said cylinder,said lTl piston dividingtsaid cylinder into first and second pressure lchambers, first conduit means connecting said iirst chamber to saidvalve, secondconduit means connecting saidV second chamber to a sourceof constant pressure, saidV piston having an orifice therethroughpermitting fiuid to flow fromsaid first chamber to said second chamberwhen there islow iiuid flow in the system, a rod connecting saidpistonto the variablerdisplacement hydraulic motor, said piston and rodadapted to move a bearing forming part `of the hydraulic motorV to varythe stroke thereof on Ia change in load on the motor.

2. Control mechanism for varying the displacement of hydraulic motors, asource of constantV fluid pressure, a uid reservoir connected to saidpressure source, a valve for controlling the flow yof fluid from saidpressure source, a first conduit means connecting the pressure `sourceto said valve, second and third Vconduit means connecting a variabledisplacementrhydraulic motor to said valve to permit the flow of fluidbetween said valve and the motor to operate they motor, fourth conduitmeans connecting said valve to said reservoir, a cylinder, a pistonreciprocally mounted in said cylinder, said piston dividing saidcylinder into two chambers, rod means Vconnecting said piston to thehydraulic motor, fifth conduit means connecting one chamber of saidcylinder with said first conduit means, sixth conduit means connectingthe other chamber to said valve, said piston being adapted to varythedisplacement of the motor.

3. Control mechanism for a variableA displacement motor of the typehaving a bearing which is movable to vary the displacement and, therebythe torque of the rnotor, comprising in combination, a source otsubstantially constant fluid pressure, a fiuid reservoir connected fluidconduit means connecting said source to said motor to permit fluid toflow into and operate said motor, second iiuid conduit means connectingsaid motor to said reservoir to permit fluid to return from saidmotor-to said source, valve means connected to said irst anl secondconduit vmeans between said source and said motor for regulatingthe'flow -offiuid between said source and said motor, a housing, means`movably mounted in said housing adapted to move the vbearing of themotor, said movable means dividing said Vhousing into separate chambers,third uid conduit means connecting one of said chambers to the source ofconstant fluid pressure, fourth iiuid conduit means connecting anotherof said chambers to said valve means.

4. The structurel dened in claim 3 in which said means mounted in saidhousing is formed with an orifice therethrough to permit uid to; flowbetween said chambers when there is low flow o fluid'between said sourceand said motor.

5. The combination defined in claim 3 in which the means movably mountedin said housing is a piston, having first and second sides,` rodV meansjoined to said first side of said pist-on to connect said piston tothemovable bearing of the motor, said second side of said piston having aysurface area greaterV than the surface area of said first side of saidpiston. Y Y

6. The combination defined in claim 5 in which said piston is Vformedwith means to permit iiuid to fiow between said chambers when there is alow ow of fluid between said source and said motor to permit full systempressure on bothsides of said piston.

7. In apparatus of the class described, a closed hydraulic circuitincluding a variable displacement hydraulic motor, a source of constantvfluid pressure, a valve for controlling the tiow of fluid in thecircuit, a piston chamber, iiuid conducting means connecting the'chamberto the valve and to the constantuid pressure source, a piston mountedfor reciprocal movement in the chamber `and dividing said chamber intotwo sections, said piston having an orifice therein adapted to permitfull system pressure to be applied to, both sides ot said piston, an

' arm connecting the piston to the hydraulic motor, said piston adaptedto automatically adjust the motor stroke on change in load on the motorwhenA a predetermined amount of fluid is permitted to flow in saidcircuit by said valve.

4 ing to said housing, antitriction means disposed between said ring andsaid pivoted bearing, a cylinder connected to said housing, a pistonhaving a low flow orifice therethrough reciprocally mounted in saidcylinder Vand dividing said cylinder intoV two chambers, a constantpressure r source, conduit means connecting said constant pressuresource to said motor, a valve connected to said conduit means to controlthe flow of liuid between said constant pressure source and said motor,`a second conduit connecting one of the cylinder chambers to said firstconduit, a third conduit connecting the other chamber of said cylinderto the valve, a rod connecting said piston to said pivoted bearing andVadapted to pivot said bearing on movement of said piston to vary thepiston stroke of said moton Y Y Y Reterences 'Cited by the ExaminerUNITED STATES PATENTS V2,328,717 9/43 Glasner 103--161 2,386,459 10/45 YHautzenroeder v 10'3--161 2,406,138 i 8/46 Ferris et al. 103-1612,407,013 9/46 Ilfield 91--47 2,673,526 3/54 Horton 103-161 2,723,5961l/55 Buchanan 103-161 2,741,993 4/56` Orshansky 103-161 2,742,879 4/56Kiester 91-49 FRED E. ENGELTHALER, Primary Examiner. SAMUEL LEVINE,Examiner.

1. IN A CONSTANT PRESSURE-VARIABLE DISPLACEMENT HYDRAULIC MOTOR SYSTEMFOR MOVING A LOAD, A VALVE IN SAID SYSTEM FOR CONTROLLING THE QUANTITYOF FLUID PER UNIT TIME WHICH MOVES IN SAID SYSTEM, A STROKE CONTROLCYLINDER, A PISTON MOUNTED FOR RECIPROCATION IN SAID CYLINDER, SAIDPISTON DIVIDING SAID CYLINDER INTO FIRST AND SECOND PRESSURE CHAMBERS,FIRST CONDUIT MEANS CONNECTING SAID FIRST CHAMBER TO SAID VALVE, SECONDCONDUIT MEANS CONNECTING SAID SECOND CHAMBER TO A SOURCE OF CONSTANTPRESSURE, SAID PISTON HAVING AN ORIFICE THERETHROUGH PERMITTING FLUID TOFLOW FROM SAID FIRST CHAMBER TO SAID SECOND CHAMBER WHEN THERE IS LOWFLUID FLOW IN THE SYSTEM, A ROD CONNECTING SAID PISTON TO THE VARIABLEDISPLACEMENT HYDRAULIC MOTOR, SAID PISTON AND ROD ADAPTED TO MOVE ABEARING FORMING PART OF THE HYDRAULIC MOTOR TO VARY THE STROKE THEREOFON A CHARGE IN LOAD ON THE MOTOR.